Asynchronous Transfer Mode (ATM) systems provide communication services over high speed, high capacity networks. The communication services can comprise various protocols for data-communication, e.g. telephone services as public switched telephone network (PSTN) and integrated services data network (ISDN), digital video/audio broadcasting, Internet services, and so on. In one type of network configuration, an ATM switch capable of transmitting and receiving data may be connected to a range of subscribers to the communication services in a point-to-multipoint architecture (“branched tree”) through a passive optical access network (PON). The requirements and specifications for the physical layer of ATM-PON systems are described in more detail in “Recommendation G.983.1” of the International Telecommunication Union (ITU-G.983.1).
In a typical network, the ATM switch is connected to the PON through an optical line terminal (OLT). Subscribers are connected to the PON through an optical network termination (ONT), which acts as an interface between the subscriber's electronic network and the optical network. Data is sent bi-directionally over the network, using different wavelengths for upstream and downstream traffic. The ATM switch transmits data frames (each containing 56 data cells, each cell containing 53 bytes) downstream to all ONTs in the network. For the upstream direction from an ONT to the ATM switch, a data frame contains 53 data cells each containing 53 bytes plus 3 bytes for traffic management. The use of an information header in the data frame and an encryption method ensures the reception of the data by only the addressed subscriber. Upstream data transmission from an ONT to the ATM switch must be synchronized with transmissions of other ONTs in the network, which is controlled by the OLT using a grant scheme by a Time Division Multiple Access (TDMA) technique. However, due to the difference in fiber length between the OLT and each of the ONTs, the propagation time delay between signals from different ONTs must be taken into account in order to avoid collisions between upstream data frames from different ONTs. Furthermore, ONTs may exhibit an equipment-related time-delay difference. Thus, for each ONT, an equalization time delay value is established during a ranging procedure. This delay value, specific for each ONT, is used to set all ONTs at an equal “virtual distance” from the OLT in order to obtain an internal timing reference for all upstream transmissions in the network.
To enhance the reliability of an optical network, an ATM-PON system can be arranged with a protection architecture. ITU-G.983.1 (appendix D) describes optional protection schemes for network architectures in which the connection of ONTs to an ATM switch is fully established through two different passive optical networks. Additionally, European patent application filed on (Lucent's reference: JNL-233-B-004), describes a PON architecture that allows a mix of protected and unprotected connections.
When in a protected or mixed architecture and a malfunction occurs in a protected connection in one of the networks, the data traffic can be switched to the other still functioning network and continue. Disadvantageously, however, due to a switching delay, a protection switch will not be hitless in the aforementioned protection architectures (i.e. without loss of data), which may severely disturb data traffic. In some protection architectures arranged with two separate networks, it is not possible to have any signals in the second of the two networks during operation of the first network due to the optical coupling arrangement of the two networks. In such a case, the second network is in a so-called cold stand-by, e.g., the laser source of the second (spare) network must be off. When a protection switch occurs, switching on the laser causes a delay and loss of data. In other protection architectures, hot stand-by (laser on) is possible, but loss of data by a malfunction can still occur, since the data traffic in the first optical network is either not synchronized with the data traffic in the second network, or the detection of the malfunction is slow in comparison with the data transmission rate of the networks.